1. Field of the Invention
This invention relates to processes of treating metal surfaces with aqueous acidic compositions to increase the resistance to corrosion of the treated metal surface, either as thus treated or after subsequent overcoating with some conventional organic based protective layer. A major object of the invention is to provide a storage stable, single package treatment that can be substantially free from hexavalent chromium but can protect metals substantially as well as the hexavalent chromium containing treatments of the prior art, or can improve the stability of treatment solutions that do contain hexavalent chromium.
2. Statement of Related Art
A very wide variety of materials have been taught in the prior art for the general purposes of the present invention, but most of them contain hexavalent chromium or other inorganic oxidizing agents which are environmentally undesirable. The specific items of related art believed by the applicant to be most nearly related to the present invention are noted below.
U.S. Pat. No. 5,089,064 of Feb. 18, 1992 to Reghi teaches a process for treating aluminum with a composition containing fluozirconic acid (H.sub.2 ZrF.sub.6), a water soluble or dispersible polymer of 3-(N-C.sub.1-4 alkyl-N-2-hydroxyethylaminomethyl)-4-hydroxystyrene, and dispersed silica. This treatment produces excellent results, but is somewhat inconvenient because the treating composition is susceptible to slow settling of the dispersed silica component. In practice, this means that for best results, at least two components, one with the silica and one without, must be stored separately and mixed shortly before use.
U.S. Pat. No. 4,963,596 of Oct. 16, 1990 to Lindert et al. teaches the use of water soluble derivatives of poly {vinyl phenol} in metal treating, including combinations of these polymer materials with dispersed silica among many other possibilities.
U.S. Pat. No. 4,921,552 of May 1, 1990 to Sander et al. teaches treating aluminum with a composition comprising fluozirconic acid, hydrofluoric acid, and a water soluble polymer.
Published European Patent Application 0 273 698 (published Jul. 6, 1988) teaches aqueous acidic treating solutions comprising trivalent metal compounds, silica, and preferably also nickel and/or fluoride ions. The counter anions for the trivalent metal cations used may be silicofluoride.
U.S. Pat. No. 4,341,558 of Jul. 27, 1982 to Yashiro et al. teaches treating metal surfaces with a composition containing a water soluble salt of zirconium and/or titanium, an inositol phosphate ester, and silica. The composition may also contain an organic binder such as poly{vinyl alcohol}.
U.S. Pat. No. 4,277,292 of Jul. 7, 1982 to Tupper teaches treating aluminum surfaces with an aqueous acidic composition containing zirconium, fluoride, and vegetable tannin.
U.S. Pat. No. 3,506,499 of Apr. 14, 1970 to Okada et al. teaches treating aluminum and zinc surfaces with an aqueous solution of chromic acid and colloidal silica.
S. M. Thomsen, "High-Silica Fluosilic Acids: Specific Reactions and the Equilibrium with Silica", Jour. Amer. Chem. Soc. 74, 1690-93 (1952), according to an abstract thereof, teaches that high-silica fluosilic acids can be prepared with any desired amount of "extra" silica up to 18% more than the composition given by the formula H.sub.2 SiF.sub.6, by dissolving hydrated silica in hydrofluoric acid. The high silica fluosilic acids show characteristic reactions with sodium salts and fluorides. A hypothesized chemical equilibrium: 4H.sup.+ +5SiF.sub.6.sup.-2 +SiO.sub.2 --3(SiF.sub.6 --SiF.sub.4).sup.-2 +2H.sub.2 O was found to have an equilibrium constant of about 100-10,000.